1. Field of The Invention
This invention generally relates to an electronic splicing device, which is used to edit videotape by carrying out electronic splicing (i.e., assemble editing) thereof, for use in a magnetic recording and/or reproducing apparatus such as a video tape recorder (VTR) and more particularly to an electronic splicing device for use in a magnetic recording and/or reproducing apparatus in which two mutually orthogonal pairs of azimuth video heads are mounted on a rotating drum having small diameter and the azimuth video heads of each pair have the same azimuth angle but the azimuth angle of the azimuth video heads of one of the pairs is different from that of the azimuth video heads of the other pair.
2. Description of The Related Art
Referring first to FIG. 9, there is shown a conventional four-head VTR provided with two mutually orthogonal pairs of azimuth heads (A, C; and B, D), which function as recording and/or reproducing heads and are equally spaced apart and mounted on a rotating drum of small diameter, so that adjacent azimuth video heads are spaced apart by 90 degrees. As illustrated in this figure, video tape is guided by stationary guide posts or poles 12 and 14 and is further pulled around a part of the peripheral surface of the drum 10 over an angular range which is a little more than 270 degrees. The diameter L of the drum 10 is selected in such a manner to meet the condition L=(2/3)M where M is head drum diameter of an ordinary existing two-head video recorder.
Further, the azimuth video heads A and C of one of the pairs have the same azimuth angle. Similarly, the azimuth video heads B and D of the other pair have the same azimuth angle. In case where this conventional four-head video tape recorder is of NTSC (National Television Systems Committee) type, each of these heads A, B, C and D and travels by one track pitch while rotates by a slightly larger than 270 degrees at about 45 revolutions per second (rps) around an axis of rotation of the drum 10. (Incidentally, in case where this conventional four-head video tape recorder is of CCIR (Commite Consultatif International des Radio-Communications) type, each of the heads A, B, C and D runs at approximately 37.5 rps.) The relative linear velocity or speed of each of the heads with respect to the tape in case of this conventional VTR is equal to that obtained in the ordinary existing VTR. Thus, this conventional four-head VTR provides tape compatibility with ordinary existing two-head VTRs. Further, two video signal frames are recorded or reproduced during three revolutions of the drum 10.
When video signals are recorded and reproduced by using the video heads of the same pair of a conventional four-head VTR, there may occur two cases with regard to video heads used to record the signals on a track and reproduce the signals from the identical track. Namely, in one of the two cases, the identical head (e.g., the head A) is used for both recording and reproducing of the signals. Hereunder, the reproducing of signals in this case will be referred to as "proper track reproduction". In the other case, a video head used to record signals on a track is different from another head used to reproduce the recorded signals from the identical track. For example, the head A is used to record video signals on a track and further the head C, which is mounted at an opposite position on the drum 10 with respect to the head A, is used to reproduce the signals from the very identical track. Hereunder, the reproducing of signals in this case will be referred to as "improper track reproduction".
In case of the "improper track reproduction", a video head used for recording video signals (hereunder sometimes referred to as a recording head) is not identical with a video head used for reproducing the video signals (hereunder sometimes referred to as a reproducing head), which, however, has the same azimuth angle as the recording head does. Further, the reproducing head is liable to differ in mechanical and electrical characteristics from the recording head. This hinders the reproduction of an image with high picture quality. Therefore, it is preferable to prevent occurrence of the "improper track reproduction". An example of a countermeasure for preventing the occurrence of the "improper track reproduction" is disclosed in, e.g., the Japanese Patent Application Provisional Publication No. 59-217256 Official Gazette (see the corresponding U.S. Pat. No. 4,605,976). In accordance with the invention disclosed therein as illustrated in FIG. 10 (B) of the instant application, the duty factor of a control signal to be recorded on a tape is adapted to vary every frame with respect to a video signal shown in FIG. 10 (A).
Namely, the duty factor is large in frames recorded by using the heads A and B, while the duty factor is small in frames recorded by using the heads C and D. Thus, it can be judged from the time of a fall in a pulse of the control signal which of the head A and the head C (alternatively, which of the head B and the head D) a reproducing head is. Thereby, the occurrence of the "improper track reproduction" can be prevented. The above described technique of preventing the occurrence of the "improper track reproduction" by making use of variation in duty factor of a control signal does not demand special signal processing. Further, a VTR employing this technique ensures tape compatibility with devices such as a Video Home System (VHS) VTR and a Super VHS (S-VHS) VTR.
Generally, in a VTR, when editing signals are recorded on videotape, an electronic splicing operation (i.e., an assemble editing operation or assembled recording operation) is frequently performed by temporarily stopping a recording operation, moving the videotape in a reverse direction over a predetermined distance and then resuming tape travel in a forward direction so as to start to record a new signal immediately after (i.e., substantially in continuance with) a previously recorded signal. As an example of techniques of improving the precision of the electronic splicing, can be cited an editing system of carrying out assembled recordings disclosed in the Japanese Patent Application Provisional Publication No. 60-179960 Official Gazette (see the corresponding U.S. Pat. No. 4,673,991). In this editing system, when the recording of video signals is once stopped, a joint indication signal (hereunder referred to simply as an indication signal) is recorded on videotape as a signal indicating change in duty factor of a control signal. Further, a beginning point for resuming the recording of a video signal on the tape is detected on the basis of the indication signal.
FIG. 11 shows signal waveforms for explaining an example of operations of this editing system. When a recording pause instruction is issued in the course of a recording operation in order to interrupt the recording of video signals and perform an electronic splicing operation (i.e., assemble editing operation), an indication signal, of which the duty factor is equal to 65% , is recorded on the tape as a control signal (see FIG. 11(B)). At that time, the recorded control signal on the tape has a pattern as illustrated in FIG. 11(C).
Further, at the time (indicated by an arrow F1) when a pulse, which follows immediately after a fourth pulse of the indication signal, of a head indicating signal shown in FIG. 11(A) falls, a video signal gets into a "muted" state, that is, the recording of the video signal is suppressed (see FIG. 11(D)). Subsequently, at the time (indicated by an arrow F2) when about 1 millisecond (mS) elapses from a fall of a fifth pulse of the indication signal, the recording of the control signal is ceased (see FIGS. 11(B), (C) and (E)). After that, the tape is rewound by a predetermined distance.
Next, when the pause (or temporary stop) in the recording of the video signal is ended and the recording operation is resumed, the rewound tape is driven and further the control signal recorded as above described is reproduced (see FIG. 11(k)). Namely, the processing of detecting the pattern of the recorded control signal on the tape (see FIG. 11(F)) is performed (see FIG. 11 (G)) and the recorded control signal is thus reproduced (see FIG. 11(H)).
Thereafter, at the time (indicated by an arrow F3) when about 1 mS elapses from a fall of a third pulse of the indication signal, a servo mode gets into a recording mode (see FIG. 11(J)). Further, the recording of video signals is started (FIG. 11(L)) at the time (indicated by an arrow F4) when a pulse, which follows immediately after a fourth pulse of the indication signal, of the head indicating signal falls (see FIG. 11(I)). As above described, the "muting" of the video signal is effected at the time F1 when the pulse, which follows immediately after the fourth pulse of the indication signal, of the head indicating signal falls, so that the electronic splicing of the video signal is carried out without loss of a video track on which the video signal is recorded. In FIG. 11, the arrows F1 and F4 indicate splice points, at each of which two adjacent records of video signals are spliced (i.e., starting positions of frames, each of which does not overlap with an adjacent frame).
Incidentally, the recording of the control signal is started (see FIG. 11(M)) at the time (indicated by an arrow F5) when a period of time corresponding to one frame elapses from the time (indicated by an arrow F3) of changing a servo mode. Thus, the recorded control signal on the tape has a pattern as illustrated in FIG. 11(N).
As described above, in each of cases of preventing the occurrence of the "improper track reproduction" and of performing the electronic splicing, the changing of the duty factor of the control signal is performed. Therefore, in order to simultaneously realize both of the prevention of the occurrence of the "improper track reproduction" and the electronic splicing, it is necessary to regulate the change in the duty factor according to control codes. Here, with regard to the duty factor of the control signal, prerequisites for simultaneously realizing both of the electronic splicing and the prevention of the "improper track reproduction" are summarized below.
(1) For the purpose of preventing the "improper track reproduction", the duty factor should be changed every frame. For example, if the duty factor for a certain frame is small, the duty factor for frames adjacent to the frame should be larger.
(2) On the other hand, for the purpose of carrying out the electronic splicing, an indication signal, which is continuous over several frames, should be recorded.
(3) In addition, for the sake of maintaining tape compatibility with other devices, VHS standards should be satisfied.
By taking into account the fact that according to the VHS standards, the duty factor for a control code "0" should range from 55% to 65%, the simplest way for simultaneously realizing both of the change of the duty factor as above described in the prerequisite (1) and the continuous indication signal as above described in the prerequisite (2) (i.e., realizing both of the electronic splicing and the prevention of the "improper track reproduction") is to change the duty factor in accordance with TABLE 1.
TABLE 1 ______________________________________ Control Control Code "0" Signal Code "1" Normal Code Indication Duty Signal Signal Signal Factor Small Large Small Large Small Large ______________________________________ % 25 30 55 58.3 61.7 65 ______________________________________
Incidentally, in case of the control code "1" signal shown in TABLE 1, VHS standards inhibit the VTR from simultaneously realizing both of the change of the duty factor as above described in the prerequisite (1) and the continuous indication signal as above described in the prerequisite (2).
The duty factor set in accordance with TABLE 1 can be discriminated by a single comparator (hereunder sometimes referred to as a "duty comparator"). In case of the control code "0" signal a reference value 56.6% is used by the "duty comparator" to judge or determine which of 55% and 58.3% is actually set as the duty factor; 60% is used to determine which of 58.3% and 61.7% the actual value of the duty factor is; and 63.4% is used to determine which of 61.7% and 65% is the actual value of the duty factor. Thus, the difference between each of the values of the duty factor and the corresponding reference value is in the order of 1%.
The above described judgement on the duty factor for preventing the "improper track reproduction" is performed immediately after a transition period in an operation of a VTR in order to decrease loss of time and improve operability of a VTR. Thus, if the comparison between the actual value of the duty factor and the reference value is made by using the above described reference value, of which the difference from the duty factor is not sufficiently large, in a transition state in which an operation of feeding the tape is not sufficiently stable, a malfunction can occur.
The present invention is accomplished to obviate the above described drawbacks of the conventional VTR.
It is accordingly an object of the present invention to provide an electronic splicing device for use in a magnetic recording and/or reproducing apparatus which can not only prevent the occurrence of the "improper track reproduction" but perform an electronic splicing of video signals well.